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Chapter 4: Problem 2

In what direction does a planet move relative to the stars when it is in direct motion? When it is in retrograde motion? How do these compare with the direction in which we see the Sun move relative to the stars?

Short Answer

Expert verified
Under direct motion, a planet moves eastward relative to the stars. In retrograde motion, a planet appears to move westward relative to the stars. The Sun, similar to planets in direct motion, also moves eastward relative to the stars.

Step by step solution

01

Understanding Direct Motion

Direct motion in astronomy is the apparent eastward motion of a planet in the sky in relation to the stars. It is eastward because this is the direction most planets move in their orbits due to the general spin direction of our solar system which was established during its formation. This is the usual motion of the planets.
02

Understanding Retrograde Motion

Retrograde motion in astronomy is the apparent westward motion of a planet in the sky in relation to the stars. This backwards or westward motion happens when from our viewpoint on Earth, the Earth passes another planet in its orbit. Because Earth鈥檚 orbit is closer to the sun and therefore faster, we view the other planet as moving backwards during the time we are passing it. Thus, retrograde motion is relatively rare.
03

The Sun鈥檚 Motion Relative to the Stars

The Sun appears to move eastward relative to the stars, making a full circuit around the sky in about a year. Therefore, it moves in the same direction as direct motion for the planets.
04

Comparison

Hence, it is observed that the most planets in direct motion and the Sun move in the same direction, i.e., eastward relative to the stars. In contrast, retrograde motion of the planets appears to be the opposite, i.e., westward.

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